Application of small geometry specimens to determine the fatigue crack growth anisotropy of Ti–6Al–4V additively manufactured for repair

Abstract

Titanium alloys are key materials for aerospace and turbine industries. Direct energy deposition (DED) is one additive manufacturing method of interest for titanium alloy repair technology. One notable feature of directed energy deposition (DED) technique is the anisotropy of the microstructure due to the build directions and local heating during layer-by-layer formations. To explore the anisotropy in fatigue crack growth (FCG) properties, single-notched, compact specimens were cut and machined from thin “repaired coupon”, i.e., half AM and half wrought Ti–6Al–4V coupon, perpendicular and parallel to the build layers; the geometry was smaller than that required by standard crack growth testing procedures. Using these small coupons, fracture properties were attained for crack growth in either parallel or perpendicular directions with respect to the build layers. Direct current potential drop (DCPD) was utilized to monitor and measured crack length during fatigue. Optical and scanning electron microscopy was performed to investigate the failure mechanisms and microstructural morphology of polished cross-section and fractured surfaces, respectively. The results show that the crack growth direction parallel to the build layer (perpendicular to the build direction) had lower FCG resistance compared to crack growth direction perpendicular to the build layer (parallel to the build direction). The difference in the FCG rate is attributed to the microstructural features and the materials growth directions. Therefore, these clearly findings show that microstructure characteristics have strong influence on fatigue crack growth behavior in both directions of growth as observed in this study.